1. Field of the Invention
This invention relates to a method compensating for the non-uniform color appearance of a color display. If there are the variations of chromaticity coordinates for each primary of every pixel in a display, the color appearance is not uniform. From the distributions of the chromaticity coordinates of original primaries, we can choose a set of virtual primaries so that the chromaticity coordinates of each virtual primary can be produced by the original primaries in every pixel. The virtual primaries are used as the primaries of the display in stead of the original primaries. Thus the color appearance can be uniform because the chromaticity coordinates of the virtual primaries are the same for all pixels. LED displays are taken as examples to show this method. From the condition that the tristimulus values of the LEDs, which are the original primaries, in a pixel are equal to those of virtual primaries, we have the formulas converting RGB video signals into modified RGB video signals which are the input video signals of the LEDs. The systems implemented according to the formulas are able to compensate for the non-uniform color appearance of the LED displays.
2. Prior Art Description
An image display usually consists of an enormous number of pixels. For a color display, a pixel is able to emit the light that contains three primary colors. For some display technologies, the color appearance is not uniform. For example, a primary color of the same luminance is to be displayed on the entire screen, different portions of the screen display different colors. Once a primary color cannot be uniformly displayed, a displayed image containing multi-color will be distorted. The non-uniform color appearance is a major deteriorate factor for light emitting diode (LED) displays. For a dot-matrix LED display, a pixel contains a set of red, green, and blue LEDs. For a scanning LED display, a raster image is created by one or many linear arrays of LEDs. Both LED displays require a large number of LEDs. Because the variations of the optical and electrical properties of LEDs are significant due to fabrication process, the homogenous quality of the color appearance of LED display is usually not good. For example, though manufacturers have sorted LEDs before shipment, greater than 10% variation of the chromaticity coordinates of the same rank LED is common for blue and green LEDs. Such a variation is not acceptable for uniform color appearance. One may further sort LEDs to reduce the variation. For a dot-matrix LED display of 800xc3x97600 resolution, it requires at least 480,000 LEDs for a single color because more LEDs of the same color may be used in a pixel to increase brightness. The number is so large that the sorting method is impractical. For a scanning LED display, though we can use the sorting method because the number of LEDs is much smaller than dot-matrix LED display, the color appearances of different displays may be different and a number of spare LEDs for an individual display is required for maintenance.
This invention relates to a method compensating for the non-uniform color appearance caused by the variations of the chromaticity coordinates of the primaries. Ideally the chromaticity coordinates of each primary of every pixel of a display are the same so that the color appearance is uniform. For the cases of LED displays, there are variations of chromaticity coordinates for each primary of every pixel and the color appearance is not uniform. Applying the color compensating method, the color appearance can be uniform. This method can also be applied to other technologies of color display. In the description of this invention, we take LED displays as examples. At first, we have to know the distributions of the chromaticity coordinates of red, green, and blue LEDs that are the light sources of the primaries. Such primaries are called the original primaries to distinguish them from the virtual primaries defined below. From the distributions, we can choose a set of three virtual primaries so that the chromaticity coordinates of each virtual primary can be produced by the original primaries in every pixel. Thus we can use the virtual primaries instead of the original primaries as the primaries of a display. The three primaries are xe2x80x9cvirtualxe2x80x9d because the three respective light sources do not really exist in the display. The light of a virtual primary usually comes from the light sources of three original primaries. As the chromaticity coordinates of the virtual primaries are the same for all pixels, the color appearance of the display is uniform and, furthermore, the color appearances of different displays are the same. Therefore we call this color compensating method as The Virtual Primary Method. Systems to implement the Virtual Primary Method are also described, which execute computations to determine the luminous intensities of each of the original primaries.
The primary color light of a color display may be emitted from many light sources. When the chromaticity coordinates of the light sources are not the same, the color appearance is not uniform. A virtual primary method was invented to compensate for the non-uniformity. For a full-color display, red, green, and blue primaries are required. From the distributions of the chromaticity coordinates of the three original primaries, we can choose a set of three virtual primaries so that the chromaticity coordinates of each virtual primary can be produced by the original primaries in every pixel. The maximum luminous intensity of each virtual primary is properly chosen so that the required luminous intensities of the original primaries to produce a virtual primary are always not negative. Furthermore the ratios among the maximum luminous intensities of virtual primaries are chosen based on a white balance condition. The virtual primaries are used as the primaries of the display, and the color appearance can be uniform because the chromaticity coordinates of the virtual primaries are the same for all pixels. LED displays are taken as an example to show this method. To quantitatively describe color appearance, the CIE 1931 color space is used. The luminous intensity and the chromaticity coordinates of every LED under a required rating operating current in an LED display are measured. The maximum luminous intensity and the chromaticity coordinates of the virtual primaries are chosen according to the measured data. From the condition that the tristimulus values of the LEDs, which are the original primaries, in a pixel are equal to those of the virtual primaries, we derive the formulas converting RGB video signals into modified RGB video signals which are the input video signals of the LEDs. A system implemented according to the formulas is able to compensate for the non-uniform color appearance. The conversion coefficients of every pixel of the display are pre-stored in memories, which depend on the maximum luminous intensities and chromaticity coordinates of the virtual primaries and the LEDs. A controller receives the RGB signals and downloads the corresponding conversion coefficients into three arithmetic logic units. The arithmetic logic units execute the computations in parallel that convert the signals according to the derived formulas.
Some displays use more than three original primaries. The Virtual Primary Method can also compensate for the non-uniform color appearances of such displays. A four-primary LED display is taken as an example. Also we must choose a set of four virtual primaries from the distributions of the chromaticity coordinates of the four original primaries. Since, there are usually only red, green, and blue video signals, such a display requires a color separation rule to obtain four video signals. From this rule together with the condition that the tristimulus values of the LEDs in a pixel are equal to those of the virtual primaries, we can derive the formulas relating the RGB signals to the four modified video signals. The other way is to convert RGB video signal into four video signals by the color separation rule first. Then, from the condition that the tristimulus values of the LEDs in a pixel are equal to those of virtual primaries, we can derive the formulas relating the four video signals to the four modified video signals. Systems implemented to convert the signals by the above methods are shown. Their operation principles are similar to that of the three-primary display shown above.